1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor laser having a quantum dot structure in an active layer and, more particularly, a method of manufacturing a semiconductor laser suitable for a light source of the high-speed long-distance optical fiber communication or the access system optical fiber communication.
2. Description of the Prior Art
As the method of converting the electric signal into the optical signal, there are the direct modulation system for modulating directly the light being output from the semiconductor laser in response to change in the modulation signal, and the external modulation system for applying the modulation to the light being output from the semiconductor laser from the outside.
Normally, the external modulation system is employed in the large-capacity optical fiber communication system, and the transmitting device that generates the optical signal consists of the quantum-well distributed-feedback type semiconductor laser for generating the continuous light and the external optical modulator for superposing the signal onto the light that is output from the laser. Assume that the quantum-well distributed-feedback type semiconductor laser is operated at a high speed under the direct modulation, the refractive index of the active layer is varied due to variation in the injection carrier density since the structure used as the active layer of the semiconductor laser is the quantum well. Thus, such a phenomenon called the chirping that shift of the oscillation wavelength is caused to restrict the transmission distance is generated.
Therefore, as described above, the external modulation system is employed normally in the commercial large-capacity optical fiber communication system. The experiment of the very high-speed/large-capacity optical fiber communication, which is in excess of 10 Gbit/s, is being made by using combination of the continuously oscillating single-mode semiconductor laser and the high-speed optical modulator.
Meanwhile, as one of important parameters in the discussions of the chirping, there is a linewidth enhancement factor. The linewidth enhancement factor α is defined by a following expression (1).                     α        =                  -                                                    ∂                                  [                                      Re                    ⁢                                          {                                              χ                        (                        N                        )                                            }                                                        ]                                                            ∂                N                                                                    ∂                                  [                                      Im                    ⁢                                          {                                              χ                        (                        N                        )                                            }                                                        ]                                                            ∂                N                                                                        (        1        )            where χ(N) is a complex susceptibility of the active layer, Re and Im are a real part and an imaginary part of χ(N) respectively, and N is a carrier density. If it is employed that the real part and the imaginary part of χ(N) are connected by the Kramers-Kronig relations and the imaginary part of χ(N) is proportional to a gain g, the expression (1) that defines the linewidth enhancement factor α can be expressed as a following expression (2).                               α          ⁡                      (                          E              ,              N                        )                          =                              -            P                    ⁢                                    ∫                              -                ∞                            ∞                        ⁢                                                                                ∂                                          g                      ⁡                                              (                                                                              E                            ′                                                    ,                          N                                                )                                                                              /                                      ∂                    N                                                                                        E                    ′                                    -                  E                                            ⁢                                                                    ⅆ                                          E                      ′                                                        /                                      ∂                                          g                      ⁡                                              (                                                                              E                            ′                                                    ,                          N                                                )                                                                                            /                                  ∂                  N                                                                                        (        2        )                            where E′ and E are the energy respectively, and P is Cauchy principal value integral.        
If the bulk structure is used as the active layer of the semiconductor laser, the linewidth enhancement factor has a value of about 4 to 6 in vicinity of the oscillation wavelength. The linewidth enhancement factor can be reduced up to about 2 by employing the quantum well structure as the active layer. It has been reported that, if the oscillation wavelength in the DFB (distributed feedback type) structure is shifted to the peak direction of the differential gain spectrum by adjusting material and composition of the quantum well and the laser structure, the linewidth enhancement factor can be reduced to a small value like 1.4 to 1.8. However, it is difficult to reduce much more this factor by using the quantum well structure. This is because, although such linewidth enhancement factor becomes 0 at the peak of the differential gain spectrum of the active layer, the peak position of the differential gain spectrum is normally placed in the absorption range of the gain spectrum in the quantum well structure and thus is displaced from the peak position of the gain spectrum.
As the semiconductor laser that is expected to reduce the linewidth enhancement factor much more than the quantum well structure semiconductor laser, the quantum-dot distributed-feedback type semiconductor laser having the quantum-dot structure in the active layer has been proposed.
FIG. 1 is a sectional view showing a structure of the quantum-dot distributed-feedback type semiconductor laser in the prior art.
An n-type AlGaAs cladding layer 12 is formed on an n-type GaAs semiconductor substrate 11, and then an i-type GaAs SCH (Separate Confinement Heterostructure) layer 13 is formed on the n-type AlGaAs cladding layer 12. An area in which quantum dots 14 are arranged in the three-dimensional direction and an area in which the quantum dots 14 are not formed are provided at a constant period alternatively in the GaAs SCH layer 13. This i-type GaAs SCH layer 13 constitutes the active layer of the semiconductor laser.
A p-type AlGaAs cladding layer 15 is formed on the GaAs SCH layer 13, and then a p-type GaAs cap layer 16 is formed on the AlGaAs cladding layer 15.
Electrode 17a, 17b are formed under the GaAs semiconductor substrate 11 and on the GaAs cap layer 16 respectively. Also, a high reflectance mirror 18 is formed on one end surface side of the GaAs SCH layer 13, and also a low reflectance mirror 19 is formed on the other end surface side of the GaAs SCH layer 13. The light is emitted through the low reflectance mirror 19.
Normally the quantum dots are formed in the strain system heteroepitaxial structure such as InAs/GaAs, or the like by utilizing the S-K (Stranski-Krastanov) mode growth that appears in the initial stage of the heteroepitaxial growth (for example, see Patent Application Publication (KOKAI) Hei 9-326506).
Next, a method of manufacturing the quantum-dot distributed-feedback type semiconductor laser in the prior art will be explained with reference to FIG. 1 and FIGS. 2A to 2G hereinafter.
First, as shown in FIG. 2A, the n-type AlGaAs cladding layer 12 of about 1400 nm thickness is formed on a (100) face of the n-type GaAs semiconductor substrate 11 by the MOVPE (Metal Organic Vapor Phase Epitaxy) method of the MBE (Molecular Beam Epitaxy) method. Then, an i-type GaAs layer 21 of about 20 nm thickness is formed on the AlGaAs cladding layer 12 by supplying TEGa (triethylgallium) and AsH3 (arsine) to the chamber. At this time, the substrate temperature is set to 620° C., for example.
Then, as shown in FIG. 2B, supply of Ga is shut off and then the substrate temperature is lowered to about 500° C. Then, an i-type InAs layer having a thickness that corresponds to one to several molecular layers is deposited by introducing the molecular beam of In into the chamber. At this time, the lattice constant of the InAs layer is slightly different from the lattice constant of the GaAs layer 21. Therefore, as shown in FIG. 2C, a large number of InAs islands 22 that are separated mutually are generated on the GaAs layer 21 by the S-K mode growth.
After the first-layer InAs islands 22 are formed In this manner, as shown in FIG. 2D, an intermediate layer 23 made of i-type GaAs and having a thickness of 2 to 3 nm is deposited on the GaAs layer 21. Thus, the InAs islands 22 are surrounded by the GaAs that has a large band gap, so that quantum dots 14 that confine the carrier three-dimensionally are formed.
Then, the formation of the InAs islands 22 and the deposition of the i-type GaAs intermediate layer 23 are repeated several times. Thus, as shown in FIG. 2E, a layer 24 having a layered quantum-dot structure in which the quantum dots 14 are arranged three-dimensionally is formed.
Then, as shown in FIG. 2F, stripe-like grooves 25 that reach the AlGaAs cladding layer 12 are formed by etching the layer 24 by virtue of the photolithography method. Then, as shown in FIG. 2G, an i-type GaAs layer is deposited on the overall surface to bury the grooves 25, so that a surface of the GaAs layer is planarized. In this manner, the GaAs SCH layer 13 is formed. An area in which the quantum dots 14 are arranged in the three-dimensional direction and an area in which the quantum dots 14 are not formed are provided at a constant period alternatively in the GaAs SCH layer 13.
Then, as shown in FIG. 1, the p-type AlGaAs cladding layer 15 and the p-type GaAs cap layer 16 are formed sequentially on the GaAs SCH layer 13. Then, the electrodes 17a, 17b and the high reflectance mirror 18 and the low reflectance mirror 19 are formed. As a result, the quantum-dot distributed-feedback type semiconductor laser is completed.
As described above, in the large-capacity optical fiber communication system, normally the transmitter is constructed by integrating the semiconductor laser and the optical modulator. However, there is such a drawback that a configuration of this system is complicated rather than the direct modulation of the semiconductor laser single body, and thus a production cost is increased.
The semiconductor laser having the quantum-dot structure has such advantages that the linewidth enhancement factor is small rather than the semiconductor laser having the quantum-well structure and also the chirping is difficult to occur even when the direct modulation is applied. However, in order to utilize the semiconductor laser in the very high-speed/large-capacity optical fiber communication system, the semiconductor laser in which the quantum dots can be formed uniformly at a higher density and which can have a larger gain is desired.